Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole is termed “logging.”
Drillers often log the borehole during the drilling process, thereby eliminating the necessity of removing or “tripping” the drilling assembly to insert a wireline logging tool to collect the data. Data collection during drilling also enables the driller to make accurate modifications or corrections as needed to steer the well or optimize drilling performance while minimizing down time. Designs for measuring conditions downhole including the movement and location of the drilling assembly contemporaneously with the drilling of the well have come to be known as “measurement-while-drilling” techniques, or “MWD”. Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as “logging while drilling” techniques, or “LWD”. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
In LWD systems, sensors in the drill string measure the desired drilling parameters and formation characteristics. While drilling is in progress these sensors continuously or intermittently transmit the information to a surface detector by some form of telemetry. Most LWD systems employ mud pulse telemetry, a system in which the drilling fluid transported by the drill string serves as the communications medium. In positive-pulse systems, a valve or other form of flow restrictor creates pressure pulses in the fluid flow by adjusting the size of a constriction in the drill string. In negative-pulse systems, a valve creates pressure pulses by releasing fluid from the interior of the drill string to the annulus. In both system types, the pressure pulses propagate at the speed of sound through the drilling fluid to the surface, where they are detected with acoustic or pressure sensors.
Systems employing mud pulse telemetry face various sources of degradation, including drilling noise, noise from motion of the drilling string within the borehole, attenuation, and noise from the circulation pumps. To combat these issues, mud pulse telemetry systems in the past have relied on fixed width pulses chosen to be long enough to support long integration times (relative to the time characteristics of the interfering noise sources), yet short enough to minimize the effect of baseline pressure drift. Indeed, in U.S. Pat. No. 5,067,114, Rorden states that the baseline drift characteristics of these systems “necessitat[e] a return-to-zero (hereafter ‘RZ’) format for reliable pulse detection”. To honor the RZ format requirement, Rorden employs various arrangements of fixed-width pulses within a fixed symbol interval to achieve combinatorial coding. However, the coding techniques put forth by Rorden may be unduly limiting.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.